Saturated and desaturated NMR response was integrated with
air-brine and air-mercury capillary pressure analysis and with
lithologic and other petrophysical analyses for cores from a carbonate
reservoir in Kansas. This integration provides guidelines for
selection of appropriate T2 cutoffs in these rocks and an understanding
of lithologic controls on permeability prediction using NMR response.
Three cores from the Mississippian reservoir, Schaben Field, Ness
County, Kansas were studied (Figure 2.1).
From these wells, 50 core plugs, representing a wide range in
porosity, permeability, and lithology were selected for detailed
investigation. Special core-analysis testing was performed on
these samples including (for most samples): routine and in situ
porosity and pore volume compressibility, routine air and in situ
Klinkenberg permeability, air-brine capillary pressure analysis
and determination of "irreducible" brine saturation,
air-mercury capillary pressure on selected samples, effective
and relative gas permeability, determination of the Archie cementation
and saturation exponents, and saturated and desaturated NMR analysis
for selected samples. Core lithologies were described and thin-sections
of representative samples were examined.

The reservoir at Schaben Field is composed primarily of dolomite
or lime mudstone-wackestone, sponge spicule-rich wackestone-packstone,
and echinoderm-rich wackestone-packstone-grainstone. Porosity
within these lithologies is generally intergranular, intercrystalline,
or moldic but may also contain a significant portion of vugs.
Grain or crystal sizes are fine to micrite size (<100um to
<2um) resulting in very fine pores. Brecciation, fracturing,
and carbonate replacement with microporous chert are common in
all lithologies. Each lithology exhibits a generally unique range
of porosity and permeability values, which together define a continuous
porosity-permeability trend. Where fracturing and vugs are present,
permeability is enhanced and the range in permeability for any
given porosity is broadened. Mercury capillary pressure analysis
shows that pore throat size for all lithologies is the dominant
control on permeability and threshold entry pressures.

Oil columns in this region are generally less than 50 feet.
Water saturations, corresponding to the capillary pressure generated
by this column, correlate well with permeability for rocks with
little or no vuggy porosity or microporous chert. Because of this
correlation, permeability prediction using both porosity and T2
is improved over prediction using porosity alone. While a causal
relationship exists between effective porosity (and pore body
size), measured by T2, and permeability, the relative influence
of effective porosity and pore body sizes appears to be small
compared to the influence of pore throats. Based on the significant
difference in the correlation between T2 and permeability for
rocks with and without vugs, it is probable that this correlation
is partially based on a correlation between pore body and pore
throat size, which can differ between lithologies exhibiting different
pore geometries. If this is correct, accurate permeability prediction
using NMR will require "calibration" of the T2-permeability
relationship for each lithology exhibiting a unique relationship
between pore throats and pore bodies. These correlations and the
constant or exponents obtained will therefore be lithology specific.
However, relationships using T2 in addition to porosity should
also provide a significant improvement over permeability prediction
using porosity alone. Where vuggy porosity is present, T2 cutoffs
appropriate for intergranular porosity do not provide good permeability
prediction. Based on these observations appropriate T2 cutoffs
for delineating effective intergranular porosity are range from
10 to 100 and increase with increasing permeability and pore throat
and body size.

Many Mississippian reservoirs exhibit high water-cuts and low
recovery efficiencies, which requires accurate reservoir characterization
and assessment for effective reservoir management. Delineation
of effective and ineffective porosity and accurate prediction
of production potential plays an important role. Conventional
logging tools provide significant data but do not generally allow
definitive identification of productive and non-productive intervals.
NMR logs provide information concerning effective porosity and
pore size, both of which can aid significantly in reservoir characterization,
but NMR response has not been evaluated against petrophysical
properties in these carbonate reservoir systems. Of particular
interest are issues concerning the selection of T2 cutoffs, permeability
prediction, and the robustness of selected parameters for the
wide range of lithologies present in these reservoirs. As part
of the Class 2 project the Schaben field has been extensively
studied to provide information for the hundreds of other small
operators who manage fields with similar characteristics.

Total Porosity.Routine helium-porosity values, measured on core plugs,
range from 4 to 26%. Petrographic analysis indicates that porosity
is dominantly intergranular or intercrystalline or moldic where
the rock is dolomitized. Locally, subaerial exposure and karstification
resulted in the development of fenestral or vuggy porosity. Porosity
values are generally highest in the grainstones and lowest in
the mudstones. In situ porosity values, measured at a net
confining stress of ~2,000 psi (13,800 kPa), are approximately
96+6% of ambient values (error represents 2 standard deviations).
NMR total porosity values agreed with helium and gravimetric fluid-filled
porosity values within the error of the various measurement methods
(+0.1 porosity percent).for 75% of the samples and was
off by approximately 1 p.u. for 25% of the samples (Figure
2.4).

"Irreducible Water" Saturation.
Air-brine and air-mercury capillary pressure curves indicate that
for many of the lithologies present in the Schaben field reservoir
there is insufficient oil column to displace water to "irreducible"
water saturation levels. Oil columns in the Schaben field range
from approximately 35-50ft, corresponding to laboratory air-brine
capillary pressures of 15-20 psi and pore entry throat diameters
of 2-3 microns. At air-brine capillary pressures of 20 psi, water
saturations (Sw20) average 26+5% higher than
those near "irreducible" brine saturation, as measured
at 1,000 psi air-brine capillary pressure (Figure
2.5). When capillary pressures are insufficient to desaturate
a rock to "irreducible" it is important to distinguish
between effective porosity as measured by NMR, which represents
the volume of the pores involved with total fluid flow, and effective
hydrocarbon porosity, which represents the fraction of the effective
porosity that is available for hydrocarbon flow. For the carbonates
studied here, permeabilities predicted using the effective porosity
must be modified to reflect the relative permeability of the actual
sample saturations. This requires the development of a correlation
between the effective hydrocarbon permeability at the appropriate
water saturation and total or absolute permeability.

Permeability.Permeability and other petrophysical
properties at the core plug scale are generally controlled by
matrix grain size and resulting pore throat diameters. Each grain
size class (e.g. mudstone, wackestone, packstone, grainstone)
exhibits a generally unique range of petrophysical properties
modified by the presence of fractures, vuggy porosity, or grain
size variations within the lithologic class. Facies comprising
multiple lithologies of differing grain size exhibit different
properties within those lithologies. Petrophysical properties
for facies that are a composite of lithologies are scale-dependent
and are a function of theproportions and architecture
within the facies. All lithologies exhibit increasing permeability
with increasing porosity and can be characterized as lying along
the same general porosity-permeability trend (Figure
2.6). Variance in permeability for any given porosity in rocks
that are not vuggy is approximately one order of magnitude and
may be primarily attributed to the influence of such lithologic
variables as the ratio and distribution of matrix and fenestral/vuggy
porosity, grain size variations, and subtle mixing or interlamination
of lithologies. Vuggy porosity appears to be isolated in mudstones
but is better connected in wackestones.

FIGURE 2.6. Cross-plot of permeability versus porosity.

Principal pore throat diameters, defined as the largest pores
that provide access to the majority of the rock porosity as measured
by air-mercury capillary pressure analysis, reveal a high degree
of correlation between these variables for these rocks (Figure
2.7).

Recognizing that pore throats are a dominant control on permeability,
raises the question as to why T2 provides such a good permeability
predictor, or improves the prediction of permeability in conjunction
with porosity, given that T2 predominantly measures pore body
properties. Correlation of the T2 peak position with pore throat
diameters, measured by air-mercury capillary pressure, indicates
that pore body size is highly correlated with pore throat size
(Figure 2.8). The accuracy of the T2-permeabilty
correlation may therefore be based on the strong association between
pore body and throat sizes. Within lithologies reflecting primarily
just grain size change, but still consisting generally of packed
spheres, this association could be anticipated to be uniform.
This is consistent with the similar T2 exponents for T2-permeability
relationships in sandstones. In lithologies exhibiting pore geometries
that are not similar to the packed sphere geometry, the relationship
between pore bodies and throats must be different. This should
be reflected in a change in the T2 exponent in the T2-permeability
equation. Figure 2.7 illustrates the difference
in correlations between T2 and permeability between pore geometries
that are predominantly intergranular and those that are vuggy.

.T2 Cutoff.The
relaxation time cutoff for distinguishing between effective and
ineffective pore sizes is often reported as 33ms for sandstones
and has ranged from 20ms to 225ms for carbonates. For the samples
analyzed in this study, to date, cutoff values, defined by the
point of divergence of the saturated and desaturated cumulative
porosity curves, increase with increasing permeability, and consequently
increasing pore body and throat size (Figure 2.9).
Rocks with vuggy porosity exhibit significantly greater cutoff
values for a given permeability than rocks with intergranular
porosity.

Saturated and desaturated NMR response was integrated with
air-brine and air-mercury capillary pressure analysis and with
lithologic and other petrophysical analyses for cores from a carbonate
reservoir in Kansas. This integration provides guidelines for
selection of appropriate T2 cutoffs in these rocks and an understanding
of lithologic controls on permeability prediction using NMR response.